Flow sensor connector

ABSTRACT

A disconnectable flow connector is formed of (a) a frusto-conical taper pin having a longitudinal bore through which the end portion of a flow tube of a mass flow sensor is inserted and secured and (b) a connector base containing a matched frusto-conical taper bore. The taper pin can be press-fit into the taper bore to form a fluid flow passage having a leakage level less than about 10 -9  scc/sec helium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the mechanical arts. In particular, it relatesto connectors for fluid flow tubes and methods for making theconnectors.

2. Discussion of Relevant Art

The measurement and control of the flow of gases is important in manyindustries. During the manufacture of semiconductors, for example, manyof the processes require a precise reaction of two or more gases undercarefully controlled conditions. Since chemical reactions occur on amolecular level, the control of mass flow is the most direct way toregulate the reactants.

There have been developed in the art a variety of instruments formeasuring the mass flow rate of gases from below 5 standard cubiccentimeters (scc) per minute to more than 500,000 scc/min. The prevalentdesign of such instruments requires that the flow of the gas be dividedinto two or more paths. Representative instruments include the flowmeters disclosed in U.S. Pat. Nos. 4,524,616 and 3,983,384, whichpatents are herein incorporated by reference.

In a typical instrument, a small flow is routed through a fluid flowtube located in a flow sensor assembly, where the flow is measured,while most of the flow is routed through a splitter section located inparallel with the flow sensor assembly.

The flow sensor assembly must be easily removable from the flow meterfor servicing, yet when in place it must be sealed from the externalenvironment to an extraordinary extent. For example, it is necessary toseal around the end portions of the fluid flow tube to avoid any crossflow between the atmosphere and the process gas. Typically, a leakbetween the process gas and the atmosphere greater than about 10⁻⁹scc/sec helium causes a problem of some type.

Because of the small size of the fluid flow tube and the severelimitation on the types of sealing materials available due to theinteraction between the sealing material and process gases, a majorproblem with existing mass flow meters is leakage of the flow sensorassembly. Existing mass flow meters use one of two methods to attach theend portions of the fluid flow tube to the remainder of the flow meter.They either use rubber O-rings alone or they use braze joints betweenthe end portions of the fluid flow tube and washers which then mate withrubber or metal O-rings.

Rubber O-rings are limited in their application, because of theirinteraction with highly reactive process gases and because they areporous to light gases at leakage levels around 10⁻⁶ scc/sec of helium.Metal O-rings are expensive and normally cannot be reused once the flowsensor assembly has been removed for servicing. Both rubber and metalO-rings form pockets which can trap impurities. Braze joints introduce anew material (braze alloy) into the flow stream which may react withsome of the highly reactive process gases used in semiconductormanufacturing.

Attempts have been made to use taper pin connectors to seal flow tubesin mass flow meters. Unfortunately, the attempts have been unsuccessfuldue to the difficulty of making very small taper pins and thecorresponding taper bores with sufficient accuracy and with surfaceshaving sufficiently smooth and complimentary finishes to form anessentially leak-free connection.

SUMMARY OF THE INVENTION

Now in accordance with the invention, it is possible to simply andeconomically manufacture taper pin connectors for mass flow meters withsufficient accuracy and with surfaces having the necessary tolerances,so that an essentially leak-free connection is made to the flow tube.The flow meter in accordance with this invention has an elongate housingwith a fluid inlet, a fluid outlet and a fluid passage between the two.The fluid passage contains a flow splitter section. The flow splittersection is operationally connected in parallel to a flow sensor assemblywhich measures the rate of fluid flow.

The flow sensor assembly contains a flow tube having a first end portionto receive fluid from the flow splitter section through a disconnectableinlet flow connector and, downstream of the inlet flow connector, asecond end portion to return the fluid to the flow splitter sectionthrough a disconnectable outlet flow connector.

The flow connectors are formed of (a) a frusto-conical taper pin havinga longitudinal bore, through which the end portion of the flow tube isinserted and secured and (b) a connector base containing a matchedfrusto-conical taper bore, such that the taper pin can be press-fit intothe taper bore to form a connection having a leakage level less thanabout 10⁻⁹ helium.

The flow sensor connector is made using a master taper pin tool and amaster connector base tool having pre-matched frusto-conical surfaces.The taper pin bore is made by inserting a relatively hard master taperpin tool into a taper pin bore in a substantially softer connector baseprecursor, with a force sufficient to deform the surface of the taperpin bore and form a taper pin bore with a frusto-conical surface havinga taper angle and a surface finish substantially identical to the taperangle and surface finish of the frusto-conical surface of the mastertaper pin tool.

The taper pin is made by inserting a relatively soft taper pin precursorinto a taper pin bore in a substantially harder master connector basetool, with a force sufficient to deform the relatively soft surface ofthe taper pin precursor and form a taper pin with a frusto-conicalsurface having a taper angle and a surface finish substantiallyidentical to the tape angle and surface finish of the frusto-conicalsurface of the master connector base tool.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration, in partial cross-section, of a flowmeter incorporating a flow connector in accordance with the invention.

FIG. 2 is an illustration, cut away, of a flow connector in accordancewith the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As required, details of illustrative embodiments of the invention aredisclosed. However, it is to be understood that these embodiments merelyexemplify the invention which may take forms different from the specificembodiments disclosed. For example, while the detailed description ofthe preferred embodiments describes flow meters, per se, it is to beunderstood that the flow meter can be combined with a valve and used tocontrol the flow of a fluid. Unless otherwise clearly indicated, theterm flow meter as used in the specification and claims includes bothflow meters and flow controllers.

And while the taper connector is described with regard to mass flowinstruments, the connectors of the present invention can be employed involume flow instruments or in any instrument where it is desired to takeadvantage of the connector's sealing ability and its resistance toreactive fluids. Structural and functional details are not necessarilyto be interpreted as limiting, but as a basis for the claims.

Referring to FIG. 1 there is shown a flow meter 10 incorporating twodisconnectable flow connectors, 11 and 12, in accordance with theinvention. The flow meter 10 has an elongate housing 13 with a fluidinlet 14, a fluid outlet 15 and a fluid passage between the two. Thefluid passage is formed by two fluid paths, one of which is formed by aflow splitter section 16 and the other by a flow sensor assembly 18.

The flow sensor assembly 18 contains a flow tube 19 connected across alongitudinal portion of the flow splitter section 16. A first endportion of the flow tube 19 is adapted to receive fluid through theinlet flow connector 11 connected with the interior of the flow splittersection 16. Downstream of the inlet flow connector 11, a second endportion of the flow tube 19 returns the fluid diverted into the sensorassembly 18 to the flow splitter section 16 through an outlet flowconnector 12.

Referring to FIG. 2, there is shown a disconnectable flow connector 20in accordance with the invention. The flow connector 20 is made of anymetal which does not react with the process fluids. The preferred metalis stainless steel, most preferably type 316 or 316L stainless steel.

The flow connector 20 has a frusto-conical taper pin 22 having anlongitudinal bore 23 therethrough. The taper pin 22 is press fit into amatched, frusto-conical taper bore 24, in a connector base 25, to form afluid flow passage having a leakage level less than about 10⁻⁹ ofhelium.

The end portion of a fluid flow tube 26 is inserted into thelongitudinal bore 23 and secured to its walls. Typically, the fluid flowtube 26 is a capillary tube having an outer diameter of from about 0.010to about 0.060 inches. In the embodiment shown in FIG. 2, the bottom ofthe taper pin 22 has been welded to the end portion of the fluid flowtube 26 to secure the fluid flow tube. Other means for securing thefluid flow tube 26 include brazing, swaging and the use of adhesives. Asupporting sealant 28, such as an epoxy resin, is used to fill the gapbetween the walls of the bore 23 and the outside walls of the fluid flowtube 26.

When the taper pin 22 is press-fit in the connector base 25 there isformed a connection having a leakage level of less than about 10⁻⁹helium. It is another advantage of the connectors in accordance with theinvention that the taper pin 22 can be disconnected from the connectorbase 25 and the connection remade many times without deteriorating theintegrity of the seal. Disconnection of the taper pin is facilitated bya removal groove 30.

It is yet another advantage of the invention that no materials otherthan the surfaces of the fluid flow tube 26 and of the taper bore 24need be in contact with the gas stream, thus eliminating any problemwhich could arise if any additional materials came into contact withreactive process gases. Additionally, the design of the flow connector20 is made such that there are no trapped gases left within the passageswhich cannot be fully purged.

Now in accordance with the invention, there has also been found aneconomical method for simply producing the matched tapered surfacesusing a frusto-conical master taper pin tool and a master connector basetool having a frusto-conical taper bore. The frusto-conical surfaces ofthe master tools are precisely machined, using conventional methods, toproduce pre-matched surfaces.

The frusto-conical master taper pin tool is formed of a relatively hardmetal, such as a hardened tool steel. Generally, the hardened metal hasa Rockwell Hardness of from about 55 C. to about 65 C.

A connector base precursor, containing a frusto-conical taper bore, isformed of substantially softer metal, such as an annealed stainlesssteel, using conventional techniques. Typically, the connector baseprecursor has a Brinell hardness of from about 140 to about 190. Thetaper angle of the surface of the taper bore is approximately equal toor slightly greater than the taper angle of the taper pin bore in thefinal connector base.

A final taper pin bore is then formed by inserting the relatively hardmaster taper pin tool into the substantially softer taper pin bore withsufficient force to deform the surface of the taper pin bore, so that itis has a taper angle and a surface finish which are substantiallyidentical to the taper angle and finish of the surface of the mastertaper pin tool. It is desirable that the master taper pin tool be ashard as practical and that the connector base precursor be as soft aspractical, to make the deformation as easy as possible, therebymaximizing the lifetime of the master taper pin tool.

Similarly, a master taper pin tool having a frusto-conical taper bore isformed of a relatively hard metal, such as a hardened tool steel.Generally, the hardened metal has a Rockwell Hardness of from about 55C. to about 65 C.

A frusto-conical taper pin precursor is formed of a substantially softermetal, such as an annealed stainless steel, using conventionaltechniques. Typically, the taper pin precursor has a Brinell hardness offrom about 140 to about 190. The taper angle of the taper pin precursoris approximately equal to or slightly smaller than the taper angle ofthe final taper pin.

A taper pin having a matched taper surface is then formed from the taperpin precursor by inserting the relatively soft taper pin precursor intothe substantially harder taper pin bore with sufficient force to deformthe surface of the taper pin so that it has a taper angle and a surfacefinish which is substantially identical to taper angle and the surfacefinish of the frusto-conical taper bore in the master connector basetool. It is desirable that the master connector base tool be as hard aspractical and that the taper pin precursor be as soft as practical, tomake the deformation as easy as possible, thereby maximizing thelifetime of the master taper pin tool.

What we claim is:
 1. A flow meter having a fluid inlet, a fluid outlet,and a fluid passage between the two, where the fluid passage comprises1)a flow splitter section, 2) a disconnectable inlet flow connectorcomprising(a) a frusto-conical inlet taper pin having a longitudinalbore therethrough and (b) a connector base containing a matchedfrusto-conical inlet taper bore in the flow splitter section, such thatthe taper pin is press-fittable in the taper bore to form a connectionhaving a leakage level less than about 10⁻⁹ scc/sec helium, 3) adisconnectable outlet flow connector comprising(a) a frusto-conicaloutlet taper pin having a longitudinal bore therethrough and (b) aconnector base containing a matched frusto-conical outlet taper bore inthe flow splitter section, such that the taper pin is press-fittable inthe taper bore to form a connection having a leakage level less thanabout 10⁻⁹ scc/sec helium, and 4) a flow sensor assembly containing aflow tube having a first end portion adapted to receive fluid from theflow splitter section, through the inlet flow connector, and, downstreamof the inlet flow connector, a second end portion adapted to return thefluid to the flow splitter section, through the outlet flow connector,wherein the first end portion of the flow tube is secured in anonrotatable fashion, and fixed, in the longitudinal bore of the inlettaper pin, the second end portion of the flow tube is secured in anonrotatable fashion, and fixed, in the longitudinal bore of the outlettaper pin, and each taper pin is press fittable into its associatedtaper bore without relative rotation between each taper pin and itsassociated taper bore.
 2. The flow meter in accordance with claim 1wherein the flow tube has a outer diameter of from about 0.010 to about0.060 inches.
 3. The flow meter in accordance with claim 1 wherein thetaper pin, the connector base and the flow tube are made of stainlesssteel.
 4. The flow meter in accordance with claim 3 wherein the endportions of the flow tube are secured to the wall structures of thelongitudinal bores by swaging, welding, brazing or affixing with anadhesive.